It is known that noise components are present in the video signal primarily in the region of the higher frequency portions thereof. It is thus possible to reduce noise by damping the high frequencies. Doing so, however, also results in a decrease of resolution, thus markedly decreasing the quality of the reproduced image. It has been proposed to transmit high frequencies essentially undamped when the video signal level is high, and to damp the signals only when the video signal is low. It is less detrimental to decrease the resolution in the dark image portions than to accept a superimposed and visible noise on the signal.
It is particularly important to reduce the signal-to-noise level when recording video signals since only a relatively low level video signal is available, derived from the video imaging tube.
The referenced U.S. Pat. No. 4,412,253 describes a circuit to decrease the noise level in a video receiver, in which a high-frequency noise component of a video image is sensed during a portion of the line retrace. A threshold value is established for suppression of high-frequency components in a noise reduction circuit. The threshold level is so controlled in dependence of the amplitude of the received signal, that high-frequency components which are less than the predetermined amplitude will not appear in the reproduced field; high-frequency components, however, which have an amplitude in excess of the threshold values are amplified. This known noise suppression circuit is comparatively complex and, under some conditions, causes deterioration of the frequency distribution of the video signal since several non-linear impedance elements must be used therein.